1. Field of the Invention
This invention relates to gyroscopes, and more specifically to a portable azimuth determining gyroscope suitable for use with weapon systems and survey equipment.
2. Description of the Prior Art
The problem of establishing the azimuth orientation of a stationary device quickly and accurately with respect to true north has received considerable attention recently. Modern military tactics require high mobility and quick reaction capability. It is necessary to determine the azimuth alignment of weapons and radar equipment quickly and accurately as soon as they are emplaced. Moreover, an accurate self contained azimuth reference is of great utility to surveyors since it permits them to operate under overcast conditions when celestial references are not available.
The conventional gyrocompass used on ships is of little use in this type of application because its settling time is much too long. Attempts to shorten this time have been partially successful, but the very short reaction times required have necessitated a different approach. Instead of configuring the gyro so that the combined effects of gravity and earth rotation precess the spin axis to lie in the meridian plane, the gyro is designed to measure two orthogonal horizontal components of earth rotation rate. The azimuth orientation of the instrument is then determined by calculating the arc tangent of the quotient of these two components.
Two basic approaches to the mechanization of this form of azimuth reference have been produced. The first approach, as generally described in U.S. Pat. No. 4,283,960 issued to Thomas R. Quermann on Aug. 18, 1981 and assigned to the Applicant's assignee, uses a gyro with a horizontal spin axis pendulously restrained about one axis and restrained in azimuth by a conventional torque feedback loop. The current in the restraining torquer is the measure of the horizontal component of earth rate perpendicular to the spin axis. To measure the required two components, this gyro assembly must be repositioned 90 degrees between measurements. The second approach uses a two degree of freedom dynamically tuned free rotor gyro mounted with its spin axis vertical. When operated in a torque feedback mode the two torquing currents are the measures of the two horizontal components of earth rate. However, in order to separate spurious gyro bias torquers from the desired rate measurements it is necessary to rotate this gyro assembly 180.degree. and repeat the measurement. The bias can be eliminated in the first implementation by reversing the direction of spin of the wheel in each azimuth position and subtracting the measurement with the wheel reversed from that with normal rotation. Wheel reversal is not effective in the second implementation because the bias is a function of wheel rotation.
It is evident that both implementations require a precision indexing mechanism to rotate the gyro assembly through a predetermined angle (90.degree. or 180.degree.). The additional bearings, gears, drive motor, stops etc. add considerably to the size, weight, complexity, and cost of the instrument and have a detrimental effect on its reliability.
Another requirement of these instruments is that they find azimuth accurately when the instrument is not perfectly leveled. This requires that the gyro be positioned internally so that the sensitive axis (axes) remain in a horizontal plane. In the second implementation this is accomplished by mounting the complete dynamically tuned gyroscope with its pickoffs and torquers on a pendulous assembly which can tilt with respect to the fixed case. Since all electrical leads required to operate the gyro must now cross an articulated joint without imposing excessive restraint, a significant increase in complexity and reduction in reliability must result.
It is obvious that the first implementation is inherently slower than the second since four sequential measurements must be made instead of two as is the case for the second. Accordingly, there is a need for an azimuth determining gyroscope which has the inherent speed of the second implementation described above, while drastically simplifying its construction by eliminating the need for an indexing drive.